1. Field of the Invention
This invention relates to a torsion type probe and a scanning probe microscope using the same.
2. Related Background Art
Thanks to the recent development of scanning tunneling microscopes (hereinafter referred to as "STMs") (G. Binnig et al., Phys. Rev. Lett., 49, 57 (1983)) adapted to directly observe the electron structure of surface atoms of conductors, the real space image of surface structure of a material can be viewed with an enhanced degree of resolution (i.e., in the order of angstroms) regardless if the material is monocrystalline or amorphous.
Currently, massive research efforts are being paid for developing scanning probe microscopes (hereinafter referred to as "SPMs") because of their effectiveness in the evaluation of microstructures. Scanning probe microscopes for detecting the surface profile of microstructures include scanning tunneling microscopes (STMs), atomic force microscopes (AFMs), magnetic force microscopes (MFMs) and scanning near-field optical microscopes (SNOMs) that respectively utilize the tunneling current, the atomic force, the magnetic force and the light obtained by bringing a probe having a microtip (microstylus) close to the sample surface to be observed.
Of these SPMs, the AFM uses a probe comprising a thin-film cantilever and a microtip mounted on the front end of the cantilever and detects the surface profile of the sample by detecting the displacement of the thin-film cantilever due to the repulsive or attractive force interacting between the microtip and the surface of the sample. It is highly effective for detecting the surface profile of a sample regardless if the sample is a conductor or insulator of electricity and hence has been studied intensively because of the wide variety of applications it provides.
The most popular technique used for the AFM to detect the surface profile of the sample is the optical lever method using a binary position sensitive detector (PSD) that detects the displacement of the optical path due to the force applied by the sample.
However, with the probe using a thin-film cantilever, the thin-film cantilever is subjected not only to vertical force but also to horizontal force (frictional force) at the same time so that the three-dimensional profile of the sample cannot be accurately traced from the detected binary PSD signal. Therefore, with detecting systems utilizing the optical lever method, the binary PSD is modified to a quarternary PSD to observe the displacement of the cantilever by dividing it into a component that is vertical relative to the surface of the sample (deflective displacement) and a component that is horizontal relative to the surface (torsional displacement) so that the three-dimensional surface profile of the sample may be observed highly accurately from the detection signal of the deflective displacement.
With a known method for detecting the displacement of the probe due to the force vertical to the surface of the sample, the probe of the thin-film cantilever is modified to a torsion type probe comprising a thin-film having a plane section rotatably supported by a pair of torsion beams (or collectively referred to as "torsion beam means"). A torsion type probe has a structure hardly affected by any torsional displacement because it is mainly displaced by the torsion on the part of the torsion beams.
Another known method of detecting displacements observes the deflection of a lever of measuring system by detecting the electric capacitance between the lever and electrodes (J. Brugger, et al., "Capacitive AFM microlever with combined integrated sensor/actuator functions", The 7th International Conference on Solid-State Sensors and Actuators (Transducers '93), p.1044 (1993)). This capacitance detection method does not depend on the shape of the lever and is adapted to detect the displacement of a torsion type probe (J. Bay, et al., "Micromachined AFM transducer with differential capacitive read-out", J. Micromech. Microeng., 5 (1995) pp.161-165). Additionally, unlike an optical lever method using an optical system including a light source, a light path and a position sensitive detector, it does not require the use of any optical system and hence is free from space-related restrictions that may arise from the alignment of the optical axis and the size of the sample to be observed.
Still additionally, it is highly adapted to the process of manufacturing semiconductor integrated circuits because the displacement sensor can be prepared by micromachining and a plurality of such sensors can be arranged on a single chip. However, the lever should be placed as close as possible to the detection electrodes while maximizing the surface area of the electrodes and the parasitic capacitance of the lever and the detection circuit should be minimized in order to achieve a satisfactorily high detection sensitivity with this method.
Generally speaking, the resonant frequency of the lever should be raised to decrease its response to the noise of external disturbance and vibrations and increase the scanning speed. However, an increased surface area of the lever and that of the detection electrodes in turn reduce the resonant frequency of the lever so that downsizing is disadvantageous for the displace detection method in terms of detection sensitivity and SN ratio.
The piezoresistance method is a method of detecting the change in the resistance of a piezoresistor formed in a thin-film cantilever to see the displacement of the latter and, like the capacitance detection method, it is also free from space-related restrictions. Piezoresistive AFMs utilizing this method are known (M. Tortonese et al., "Atomic force microscopy using a piezoresistive cantilever", Transducers '91, pp.448-451 (1991)).
A piezoresistive AFM provides a high detection sensitivity and a resolution in the order of angstroms.
A method of preparing a piezoresistive cantilever for a piezoresistive AFM will be described by referring to FIGS. 1A through 1C of the accompanying drawings.
The starting material is an SOI (silicon on insulator) wafer formed by arranging a silicon layer 504 on a silicon wafer 502 with a silicon dioxide separation layer 503 interposed therebetween (FIG. 1A) and then another silicon dioxide layer 505 is formed on the rear surface of the SOI wafer.
Then, a piezoresistor layer 506 is formed in the surface of the silicon layer 504 by implanting boron ions into the silicon layer by means of an ion implantation technique and a cantilever is formed from the silicon layer by patterning, using photolithography and etching. An opening 507 is formed through the silicon dioxide layer 505 on the rear surface of the wafer also by patterning.
Thereafter, a silicon dioxide thin film 508 is formed on the surface of the cantilever. Subsequently, a contact hole 509 is formed and then Al electrodes 510 are formed to detect the change in the resistance of the piezoresistive layer (FIG. 1B).
After applying polyimide to the surface of the wafer by spinning to form a polyimide layer and hardening it, the wafer is subjected to crystal anisotropic etching from the opening on the rear surface of the wafer, using an EDP (ethylene diamine pyrocathecol) aqueous solution, to produce a support member 511. Finally, the separation layer 503 and the polyimide layer are removed (FIG. 1C) to produce a piezoresistive cantilever having a plan view as shown in FIG. 2.
Such a piezoresistive cantilever shows an excellent reproducibility because it can be prepared by utilizing a process of manufacturing a semiconductor integrated circuit. Therefore, it is possible to provide a cantilever carrying a plurality of displacement sensors on a single chip.
Additionally, the piezoresistive method is less subjected to restrictions to downsizing of the cantilever when compared with the capacitance detection method and it is possible to provide a cantilever having a resonant frequency of more than 10 kHz with a detection sensitivity comparable to that of an optical lever method.
However, known piezoresistive AFMs utilize a thin-film cantilever and hence, when the surface of a sample is scanned by the cantilever to detect the profile of the sample, the signal representing the change in the resistance of the piezoresistor actually includes both the change due to the deflectional displacement of the cantilever and the change due to the torsional displacement of the cantilever.
Therefore, the piezoresistive cantilever of any known piezoresistive AFM cannot discriminate the change in the resistance due to the deflectional displacement of the cantilever from the change due to its torsional displacement to accurately detect the surface profile of a sample.